US allies around the globe have begun to contemplate a future in which America plays a more restrained role. Here in Australia, we’re acutely conscious of the recent surge of interest in the future of ANZUS. But media reports also tell of a sudden interest among some German commentators in the possibility of an independent European nuclear arsenal, or even an indigenous German one. And northeast Asian allies are contemplating both possible reductions in US forces stationed in their countries and the withdrawal of the US nuclear umbrella.

In short, US assurance policies are struggling. To see why, let’s step back to think about what assurance is. It’s the confidence that an ally will provide assistance when serious national interests are threatened. So there are two factors that contribute to that level of confidence: the assuring state’s capability to provide actual assistance, and its resolve to do so when needed. If we wanted to represent assurance as a mathematical equation, it would be written as A = C x R, (Assurance equals Capabilities multiplied by Resolve). The equation is one of multiplication rather than addition because if either of the variables is zero, so is assurance.

Of course, assurance—like deterrence—is in the eye of the beholder. And typically the beholder’s interested not merely in some level of capability, but in a margin of superiority over a potential adversary. Similarly, the beholder’s interested not merely in some level of resolve, but in that level of resolve which suggests that vital national interests can be safely entrusted to the hands of a foreign power. So we might rewrite the equation as A = PMC x PHR (Assurance equals the Perceived Margin of Capability multiplied by Perceived High-level Resolve).

And here’s where the problems start to become clearer. As other great powers have risen, and are developing and modernising their military capabilities and unfolding anti-access and area denial plans for their immediate environs, it’s become harder for the US to retail a story about its margin of capability over potential adversaries. Over the past decade the need to tell that story has forced the US into its Air-Sea Battle doctrine and, subsequently, the Third Offset. Neither are especially convincing, so on the capabilities side of the equation the overall assessment is one of relative US slippage. The upshot is that US allies proximate to potentially hostile great powers are more anxious about Washington’s ability to save them.

There’s a particular wrinkle on that side of the equation in relation to nuclear weapons. The American determination to decrease the profile of nuclear weapons in its own strategic posture has increased anxieties among its allies covered by the US nuclear umbrella. That’s because the margin of conventional superiority of US forces seems to be narrowing over time, as other great powers modernise their militaries. But it’s also because some traditionally-constrained actors, like North Korea, are moving closer to a point where they could use nuclear weapons against the US homeland if Washington was to intervene on its ally’s behalf. (That’s one factor driving US and allied interest in ballistic missile defences.)

So, the capabilities factor of the equation has its problems. But with Donald Trump’s election as US president, we now have a new bout of allied anxiety about the resolve factor as well. Trump campaigned on the notion of ‘America first’. He weakened US declaratory policy about alliances, portraying US commitments as optional, and depicting security partnerships as protection rackets. He ruminated about whether it might be better for Japan and South Korea to build their own nuclear weapons, rather than rely upon US extended nuclear deterrence. Nationalism and unilateralism trumped internationalism and consultation.

So allies now worry not only about a shrinking margin of US capability, but also about the US commitment to protect their vital interests in a world of contested multipolarity. And that’s driving those allies to consider a range of strategic options that weren’t even on the table in earlier days.

Is there a path back to the earlier condition of robust assurance? Possibly, but the incoming administration’s going to have its work cut out on both factors in the assurance equation. Restoring US declaratory policy probably involves telling a new ordering story about the US role in the world during an age of contested multipolarity and domestic priorities. That’s a big ask. Restoring a US margin of capability over potential adversaries might be an even bigger one.

The obvious answer is that US allies are going to have to bring more to the party, and to depend less upon Washington to safeguard their interests. Their doing so would help to encourage the US to stick around—after all, assurance is a two-way street. But a set of gamechanger strategic policies in Asia might well lie down that path.. We should brace ourselves for a roller-coaster rise ahead.